Comminuted alloy



Patented June 19, 1934 UNITED STATES PATENT OFFICE No Drawing. Application August 5, 1933, Serial No. 683,910

8 Claims.

This invention relates to comminuted metals or alloys which are intended to be mixed with another metal or alloy which, like mercury or gallium, is liquid at or near ordinary temperatures and. can unite with the former to produce a plastic mixture that will harden more or less rapidly into a metallic body that is essentially solid and rigid. Among such alloys are those known as dental amalgam alloys which are used 10 in repairing teeth and also for making models or dies to assist in shaping tooth-inlays cast from alloys of gold and other metals.

An object of this invention is to provide a dental amalgam alloy having particles of such sizes and shapes that a tooth cavity can be readily and completely sealed without the formation of any voids within the amalgam or between the amalgam and the tooth, that is, providing a tight joint between the filling and the cavity walls so as to prevent subsequent decay in the repaired part of the tooth.

While amalgam alloys are known which expand slightly during the hardening process, it has been found in the research leading up to this 26 invention that an expanding alloy by no means guarantees the absence of voids and channels between the surface of the filling and the contiguous surface of the cavity. Such-an expanding alloy merely assists in preventing a well made 30 joint from leaking later by separationof the contacting surfaces. I

In the first place it should be pointed out that when a dental amalgam is ready to be packed into a tooth cavity it is not essentially a solution of the alloy in mercury, but is essentially a suspension of minute discrete particles of unchanged alloy in a solution of the component metals in mercury.

In accordance with this invention, the particles of the alloy suspended in the mercurial so- "lution are small enough and so shaped and graded that they can be fitted closely together to yield I a fine grained, dense structure when the amalgam has become fully hardened. Unless this shape and size requirement is fulfilled, no amount of expansion, strength and freedom from plastic flow-can be expected to yield a non-leaking joint .between the amalgam and the tooth.

Heretofore' the only amalgam alloys offered to '50 the dental profession have been supplied in the form of irregularly shaped, more or less curled, rough chips that are permeated with countless .minute cracks and fissures, the chips varying in size and shape from fine filings to rather wide, thin shavings The uncombined mercury that initially occupies these voids in sucn material is distributed throughout the filling in comparatively large units, and the diffusion of this excess mercury into the grains of the alloy results in an undesired porosity of the amalgam and also prolongs the time required for the amalgam to reach chemical and physical equilibrium. Such particles being rough and jagged, cannot respond to the operation of condensation to be packed into the minimum volume they should occupy to produce the most satisfactory filling. Failure to attain this minimum volume results in undesired voids' and channels among the alloy particles and between the amalgam and the tooth.

Moreover, in the processes heretofore used for making fully annealed dental amalgam alloys the chips become contaminated with a film, usually of a leaden bluish gray color that differs substantially from the natural color of the virgin alloy in the ingots from which the chips are cut. 175.-

The presence of such a contaminating film interferes with the union "of the alloy with the mercury.

In order to overcome these defects in dental amalgam alloys heretofore supplied, as well as to gain other advantages which will become apparent later, this invention provides the dentist with an alloy in the form of uncontaminated smooth microgranules substantially free from cracks and fissures, and so shaped and graded '35 in size that they will yield an amalgam that can not only be packed into the tooth cavity to provide a complete filling and sealing of the cavity, but also can be prepared more quickly by the dentist.

The surfaces of the microgranules of this invention are substantially convex, so that granules are shaped more like lumps of anthracite pea coal than like chips or metal cut by tools. Preferably the length of a granule should not be greater than three times its width or thickness. The surfaces are also smooth and brilliant like the surfaces of freshly fractured brittle metallic crystals, and are of the same color as the virgin alloy in the ingots from which the granules are produced, unmodified by tarnish from oxides or other impurities. They aresubstantially free from fissures and roughness usually found on the concave surfaces of curled metallic chips, and they are also free from permeating cracks and crevices. Sharp corners and edges are also avoided, so as to be relatively free from rough, jagged projections, which in other amalgam alloys interfere with reliable measurement of quantity of alloy by a dispensing apparatus, and

not more than 2% zinc.

= means of a centrifugal casting machine.

which interfere also with packing the amalgam into intimate adaptation with the walls of the tooth cavity and with producing a fine grained structure.

The granules of this invention are graded in size, preferably within a specified tolerance designed to approach within commercially practi-.

cable limits the elimination of voids in the filling While securing at the same time the proper expansion characteristic during the hardening of the amalgam. Preferablythe granules should not be less than microns nor more than will microns in any dimension andaveraging about microns as the longest dimension. As previ= ously stated the length of each granule should not be greater than three times its width or thickness. The proper sizes for the material may, if desired, be selected by an elutriator. Particles that will be carried away by a vertical stream of water flowing upwardly at the rate of 2 cm. per min. are too small to be used satisfactorily for the comminuted alloy of this in-= vention while particles that fail to be carried up-.

wardly by a stream of waterflowing at the rate .of cm. per min. should be discarded as being too large. Particles of a size determined by these two limits will be found quite satisfactory.

The chemical composition of the alloy of this invention may vary within wide limits depending upon the purpose for which the alloy is to be used. Dental amalgam alloys of the better grades intended for filling teeth in the mouth usually contain not less than silver, not less than 25% tin, not more than 6% copper and Amalgam alloys intended for such uses as making models and dies may contain considerably. less silver and considerably more of the other metals. For tooth restorations in the mouth I prefer to use an alloy of about 67% to 70% silver, from 25% to 29% tin; from 3% to 5% copper and less than 1% zinc. To these .metals may be added small amounts of other metals such as beryllium, gallium, indium, germanium, or palladium in order to modify the properties of either the alloy or the amalgam, especially to modify the color or the expansion during hardening.

Granules having the improved characteristics described in the foregoing are preferably produced by cutting ingots of the alloy into chips of the proper thickness, grinding these chips in a suitable grinding device, sorting the resulting granules by a suitable elutriating device, annealing the granules under suitable temperaturetime conditions to prevent appreciable change in purity, are alloyed by melting in an electric furnace while they are thoroughly protected from contamination, oxidation for example, by the use of a reducing mixture of hydrocarbon gases. The molten alloy is .then cast into tubular ingots by During the casting and until the alloy is cold, the alloy is protected by the reducing atmosphere of hydrocarbon gases. Such a process produces ingots entirely free from dross, impurities, pipes, or

- cracks. For convenience in'reducing the alloy to chips, as well as for obtaining clean, sound metal,

these ingots are cast in the form of tubes about 1 Chips, preferably 10 to 30 microns thick, are cut accaoee from these ingots by a machine in which the cutting is done under the surface of a cooling and lubricating liquid (which may be water), in order to prevent appreciable warming of either the alloy. or the cutting tool, and to prevent contamination of the chips by oxidation, which always occurs when the cutting is done in air as is usual in processes heretofore employed. Since dental alloys of the above described composition are brittle, the chips are traversed by many fine cracks, which form starting places for splitting the granules apart by a grinding operation which leaves clean and brilliant cleavage surfaces. The

grinding is also preferably done under the surface of a cooling fluid. The subsequent annealing which follows the grinding is done under a reducing solution to preserve the natural color of the virgin alloy. The annealed granules are washed free from the annealing solution and are sorted by elutriation as previously described, and after being dried, the selected particles are ready for use as a dental alloy. Particles so prepared and having the specified size limits above given, after mixture with mercury in a suitable amount, can be readily packed into theminimum volume the particles will occupy, so that the tooth cavity will be completely sealed without any voids or fissures which would promote additional tooth decay.

The above described process for producing comminuted particles is not claimed herein but constitutes a part of the invention set forth and claimed in my copending application Serial No. 719,149, filed April 5, 1934 on Process for manufacturing comminuted metals.

What is claimed is:

l. An alloy for dental amalgams comprising finely divided solid particles, substantially all of said particles being hard, dense, and free from cracks or jagged projections, substantially all of said particles having a length less than microns with a width and thickness well within the same order of magnitude as the length, said alloy having a composition such that when the alloy i combinedwith mercury a plastic mixture results metallic body.

2. An alloy for dental amalgams comprising finely dividedsolid particle's, substantially all of said particles being hard, dense, and free from cracks or jagged projections, substantially all said particles having a length less than 100 microns with a width and thickness well within the same order of magnitude as the length; said alloy having a composition such that when the alloy is combined with mercury the resulting amalgam will expand slightly during hardening.

3. An alloy in accordance withclaim 2 in which said alloy is composed mainly of silver with smaller amounts of tin and copper, 7*

v 4. An alloy in accordance with claim 2 in which said alloy comprises 67% to 70% silver, 25% to 29%tin, 3% to 5% copper and less than 1% zinc.

5. An alloy for dental amalgams comprising finely divided solid particles, substantially all of said particles being hard, dense, and free from cracks or jagged projections, said particles having an average length of about .30 microns with a width and thickness well within the same order ill) -of magnitude as the.length.

'6. A comminuted alloy for dental amalgams, 1'45 substantially all the particles of which are hard, dense, and free from cracks or crevices, substantially all of said particles having a length less than 100 microns but more than 10 microns, substantially all the particles having a width and hich will harden rapidly into a solid 120 microns and having width and thickness at least one-third the length, said particles being free from jagged corners and cracks orcrevlces.

8. A comminuted fully annealed alloy in accordance with claim '7 in which substantially all the particles are untarnished and possess the natural color of the virginalloy.

ARTHUR W. GRAY. 

